The present invention relates generally to heat sinks for electronic components and more particularly to a mounting assembly of the kind that has tool-free heat sink retention and that accommodates a wide range of tolerances in a Z-axis stackup of a component.
Modern electronic appliances such as computers have many hundreds of integrated circuits and other electronic components, most of which are mounted on printed circuit boards. Many of these components generate heat during normal operation. Components that are relatively big or that have a relatively small number of functions in relation to their size, as for example individual transistors or small-scale integrated circuits, usually dissipate all their heat without a heat sink. The large physical sizes of such components, especially as compared with their active portions, limits their density on a circuit board sufficiently that there is enough room for any heat sinks that may be needed. Accordingly, any component that needs assistance in dissipating heat can have a heat sink of its own.
The term “heat sink” as used herein generally refers to a passive device, for example an extruded aluminum plate with a plurality of fins, which is thermally coupled to an electronic component to absorb heat from the component. The heat sink dissipates this heat into the air by convection.
One widely used method of increasing the speed of an electronic circuit is to reduce the lengths of the connecting wires. In part, this is accomplished by abandoning the older practice of enclosing each integrated circuit chip in a separate package in favor of mounting many chips next to each other on a single substrate. Such an assembly of chips and substrate is commonly referred to as a multi-chip module (“MCM”). However, since the chips are typically not all identical, the upper surface of these chips are not necessarily coplanar. In addition, the space required by the mounting hardware for an individual heat sink usually requires through holes in the printed circuit board. This negatively impacts routed traces in the area around the component being heat sinked.
Another known heat sink system uses shoulder screws and springs to maintain the appropriate compressive force to maintain the required thermal bond between the heat sink and an upper surface of a chip or other component on the substrate. However, such shoulder screws and springs can unevenly load the heat sink as the springs are torqued down. They also have many pieces, and require tools to install or service the component under the heat sink.
Simple spring or wireform clips may work well for small heat sinks, but are very limited in the force they apply to the heat sink because a user must be able to compress them. Therefore, they are limited to smaller heat sinks.
Those systems that have some type of clam shell/leaf spring arrangement require height above the heat sink. However, this reduces the height available for fins and limits performance in constrained areas.
One example of a compound wireform 100 is depicted in FIG. 1. The wireform 100 secures heat sink 102. A first end 104 of the wireform 100 has a pivot axis about which the wireform 100 pivots from an open position (not shown) to a closed position (depicted in FIG. 1). A second end 106 of the wireform 100 (or a section fo the wireform 100 in the vicinity of the second end 106) may be held in place by secondary clamp 108. This design allows more force to be applied to the heat sink. However, to generate the needed 60–100 lbs. of force, the wireform must be made of a large diameter (>4 mm) to withstand the stresses. Unfortunately, this gives the wireform a working deflection of only about 0.6 mm, not nearly enough to accommodate the ±0.4 mm of stack height variation and still maintain the 60 lbs's +/−15 lbs. target load on the component.
There have been many attempts to solve the problem of dissipating heat developed by high-power integrated circuit chips in an MCM. Some of these solutions are mechanically complex, or are expensive, or make it difficult or impossible to rework or service the MCM. For these and other reasons, none of the prior approaches has adequately solved the problem.
From the foregoing it is apparent that there is still a need for an apparatus and method that allows more force to be applied to the heat sink than in the prior art. Such need is for tool-free heat sink retention and accommodation of a wide range of tolerances in the Z-axis stackup of, for example, components on a printed circuit board.